This invention relates to high performance semiconductor device packaging.
Semiconductor devices increasingly require lower cost packaging with higher thermal and electrical performance. A common package used for high performance devices is the Plastic Ball Grid Array (xe2x80x9cPBGAxe2x80x9d). The PBGA is a surface mount package that can provide higher thermal and electrical performance, and a lower thickness profile and a smaller footprint, as compared to leadframe based surface mount packages such as Plastic Quad Flat Package (xe2x80x9cPQFPxe2x80x9d) and others. Improvements are sought in the structure and design of the package, to provide increased thermal and electrical performance and to maintain the established footprint and thickness characteristics of standard PBGAs.
In conventional PBGAs a small fraction of the heat generated by the semiconductor device dissipates to the ambient through the molding compound, principally at the upper surface of the package, and, to a much lesser extent, through the sides. Most of the heat that is generated by the semiconductor device in standard PBGAs is conducted through the solder balls to the product board, and the board acts as a heat sink.
Various approaches have been employed or suggested for increasing power dissipation from PBGAs. For example, power dissipation to the ambient can be increased by blowing air over the package; but cost considerations or space limitations may make such air cooling approaches impractical. And, for example, power dissipation can be increased by increasing the number of solder balls between the package and the board, and, particularly, by increasing the number of balls directly beneath the device; and by using a laminate substrate having multiple metal layers. These approaches require increases in package dimensions and changes in the package structure.
In another approach to increasing power dissipation from PBGAs, often referred to as xe2x80x9cThermally Enhanced PBGAxe2x80x9d or TEPBGA, a partially embedded metal heat spreader is employed. The partially embedded metal heat spreader includes an upper generally planar portion having a top surface free of the molding compound and exposed to ambient; and embedded portions, which may be referred to as supporting arms, extending from the upper portion downward to the substrate and attached at the lower ends to the upper or die support surface of the substrate.
Conventionally, TEPBGAs having partially embedded beat spreaders are formed generally as follows. A completed ball grid array is provided, with the die attached on the die support surface of the substrate and connected to the substrate, e.g. by wire bonds. A heat spreader is placed on the support surface of the substrate over the die, with the heat spreader supporting arms rigidly affixed onto the supporting surface using, for example, a cement or adhesive, such as an epoxy cement or adhesive or glue. Then this assembly is placed over a mold cavity in an injection mold, so that the upper surface of the upper portion of the heat sink is at the bottom of the mold cavity and the substrate surface contacts the edges of the upper opening of the mold cavity. Then the molding compound is injected into the cavity, where it encloses the die and the wire bonds and the supporting arms of the heat spreader and fills the space between the upper surface of the die and the upper portion of the heat spreader. The molding material hardens to form the mold cap, and the completed assembly is released from the mold.
Manufacturing defects can occur in the conventional process for forming a TEPBGA with a partially embedded heat spreader, as a result of stress on the rigid attachment of the supporting arms to the substrate surface. Particularly, apparently, flexing of the supporting arms during processing can result in breakage of the substrate. Moreover, because there is a significant difference in the coefficient of thermal expansion of the material of the heat spreader, which is a metal such as copper, and the coefficient of thermal expansion of the substrate material, stresses develop during temperature cycling when the device is in use, and such stresses can create cracks in the package substrate, leading to package and device failure. We have discovered that such manufacturing defects and stress effects can be avoided by either performing the injection molding process without affixing the supporting arms to the substrate at all, or by employing a resilient material such as an elastomeric adhesive to affix one or more of the supporting arms to the substrate surface prior to injection molding. The resulting package is less subject to thermal cycling-relayed stress and stress damage, because relief is provided between the supporting arms of the heat spreader and the substrate. This is provided according to the invention because there is either no direct attachment at all between the heat spreader support arms and the substrate or, if there is direct attachment of one or more of the support arms to the substrate, it is a resilient joint.
Accordingly, in one general aspect the invention features a method for manufacturing a plastic ball grid array package, by placing a heat spreader having an upper portion and a plurality of support arms into a mold cavity; placing over the mold cavity a ball grid array including a semiconductor die mounted on a support surface of a substrate and connected to the substrate, such that lower ends of the support arms contact the support surface of the substrate peripheral to the die; injecting molding material into the cavity to form the molding cap; and permitting the molding material to harden to form a mold cap.
In another general aspect the invention features a process for forming a TEPBGA with a partially embedded heat spreader, by placing a heat spreader having an upper portion and a plurality of support arms onto the die support surface of a substrate such that at least one of the supporting arms of the heat spreader is affixed to the substrate using a resilient fixative such as an elastomeric adhesive; placing a mold cavity over the heat spreader; injecting the molding material into the cavity; and permitting the molding material to harden to form the mold cap. The resilient fixative holds the heat spreader in the appropriate position in relation to the substrate during injection of the molding material, and as the molding material hardens to form the mold cap the partially embedded heat spreader becomes fixed in the appropriate position in relation to the substrate.
In another general aspect the invention features a plastic ball grid array semiconductor package including a metal heat spreader having supporting arms embedded in the molding cap, in which the embedded supporting arms are free of direct rigid affixation to the substrate; or in which any supporting arm that is affixed to the substrate is affixed using a resilient material such as an elastomeric adhesive.
In some embodiments the heat spreader is constructed of metal and, in particular embodiments, the heat spreader is constructed of copper. In some embodiments the heat spreader has four supporting arms, configured so that their lower ends contact the substrate surface in a generally rectangular, preferably generally square, array. In some embodiments the resilient material has an elastic modulus in the range 0.5 MPa to 100 MPa, preferably in the range 1 MPa to 10 MPa, and in particular embodiments the resilient material has an elastic modulus of 5.5 MPa. In some embodiments the resilient material is an elastomeric adhesive, for example a silicon adhesive such as the adhesive commercially available as Dow Corning 7920.
In some embodiments the elements of the package are selected so that the overall dimensions of the package are within standard specifications (and, particularly, so that the overall package thickness is about the same as or less than that of standard PBGA packages). Particularly, for example, in some embodiments the thicknesses of the die plus die attach epoxy, the wire bond loop height and the wire-to-mold clearance are determined so that the height from the substrate to the top of the package (that is, the sum of the overall mold cap thickness plus the thickness of the heat spreader and the thickness of the heat spreader adhesive) is no more than 1.17 mm. And, for example, in some embodiments the thicknesses of the portions of elements situated between the semiconductor device and the heat spreaderxe2x80x94that is, the elements that lie in the critical thermal pathxe2x80x94are determined so as to minimize the length of the critical thermal path.
Particularly, for example, in some embodiments at least part of the molding material between the die and the upper portion of the heat spreader is made as thin as is practicable while avoiding contact between the upper surface of the die and the under surface of the heat spreader, and the upper portion of the heat spreader is accordingly thickened by downward protrusion of the lower surface. As a result, there is an increased proportion of metal in the path between the semiconductor device and the upper surface of the package, so that the combined thermal resistance on the critical heat path from the die to overlying ambient is reduced. And, for example, in some embodiments at least some portion of the volume between the die and the upper portion of the heat spreader is occupied by a material having a lower thermal resistivity than the molding material.
In some particularly preferred embodiments a black copper oxide or a chemically roughened copper under surface of the heat spreader is employed in order to enhance the adhesion between the bottom of the heat spreader and the mold compound. The under surface of the heat spreader is treated to provide the black copper oxide layer or chemically roughened surface before injection of the molding material. Where a black oxide is employed, it can be formed by, for example, exposing the copper surface to NaClO2 for a time sufficient to form the layer to a desired thickness, preferably in the range about 3 um to 15 um and in particular embodiments about 7 um thick. Or, where a chemically roughened copper surface is employed, it can be formed by a micro-etching process such as a conventional H2SO4xe2x80x94H2O2 process or other chemical process, to provide the desired roughness, preferably in the range 0.5 um to 1.0 um and in particular embodiments about 0.5 um.
The invention can provide excellent power dissipation in a package more reliably manufacturable than conventional packages, and less likely to fail during thermal cycling in use than conventional packages.